The long range purpose of this project is to investigate the role of type C retroviruses as an etiologic agent and a vector of genetic information for neoplasia, and the immunologic responses of the host to tumor associated antigens. The topics of current interest are: 1) Further study of the role of a B-tropic virus (11A-MuLV) isolated from an 11A variant cell line in transmitting immunity against Meth A tumor transplantation. 2) Isolation and characterization of a novel dual-tropic virus with B-tropic and xenotropic properties. 3) Clonal purification and further characterization of xenotropic viruses (isolated from SJL/J mice and Abelson tumor of BALB/c mice), a dual tropic virus (isolated from Abelson tumor) and amphotropic viruses (isolated from RBL-5 and YAC tumors). 4) Further studies on the natural killer (NK) cell activity associated with reticulum cell neoplasms (RCN) of SJL/J mice. 5) Immunopotentiating effect of a glucan (lentinan) on virus infections and virus-induced tumor transplantation resistance. 6) Further immunologic and genetic studies on the transplantation resistance of established murine trophoblast cultures.